The invention relates to a zoom system, in particular, a zoom system for an illumination device of a microlithographic projection exposure system.
The purpose of the illumination devices employed on microlithographic projection exposure systems is uniformly illuminating a reticle arranged in the object plane of a projection lens that follows the reticle in the optical train in a manner that has been accurately adapted to suit the optical properties of the projection lens. The illumination should be telecentric in order that the directed foci of all points in the plane of the reticle will be centered on the entrance pupil of the projection lens as accurately as possible. It may also be desirable to provide partially coherent illumination for which the extent to which that entrance pupil is filled will be variable and adjustable. Zoom systems may be employed for varying the degree of coherence of the illumination. In order to allow arriving at a close approximation to the limits of resolution of the optical projection during the photolithographic micropatterning process, the illumination is frequently optimized to suit the patterns on the individual layouts by creating various illumination modes, for example, annular illumination or quadrupole illumination. Devices, such as conical or pyramidal axicons, may be incorporated into zoom systems for that purpose, since there is a demand for high illumination efficiency in order to allow utilizing the light outputs of the light sources employed for fabricating microdevices with the least possible light losses.
Illumination devices that meet that demand well are disclosed in, for example, European Patent EP 0 747 772, German Patent DE 44 21 053, and European Patent EP 0 687 956. In the case of the illumination system of European Patent EP 0 747 772, the zoom system has a plurality of lenses that are arranged along an optical axis and define an object plane and an image plane that is a Fourier transform of the object plane. Two of its lenses are movable lenses that are movable along the optical axis when setting zooming positions of the zoom system in order to vary the size of an illuminated area on the image plane. Graticular, diffractive, optical elements bearing two-dimensional, graticular patterns are arranged in both the object plane and the exit pupil of the zoom lens. That arrangement suitably increases the light guidance factor, where that graticular optical element that is arranged in the object plane, together with the zoom system, introduce a small portion of the light guidance factor and the graticular optical element arranged in the image plane both generates the major share of the light guidance factor and adapts the illumination to suit the size of the illuminated field, for example, the rectangular entrance surface of a rod-shaped light integrator that follows it in the optical train. The graticular elements may also be called raster elements or rastered elements. The zoom system has a zoom ratio (expansion ratio) of three in order that partially coherent illumination having degrees of coherence ranging from 0.3 to 0.9 may be set.
Employment of a zoom system on the illumination device of a wafer stepper in order to allow adjusting the degree of coherence of the illumination with no light losses is known from U.S. Pat. No. 5,237,367.
An afocal zoom system for illuminating wafer steppers that also allows adaptation of the degree of coherence of the illumination with low light losses is known from U.S. Pat. No. 5,245,384.
An illumination system having an afocal optical system that serves as a beam expander in order to convert an incident light beam having parallel rays into an exiting light beam that has a larger cross-sectional area and also has parallel light rays is known from U.S. Pat. No. 5,955,243. The system has a first lens group having a negative refractive power on its entrance end that is followed by a second lens group having a high positive refractive power that jointly focus the incoming light beam onto a focal plane that is located at a distance behind the second lens group. A third lens group having a positive refractive power that collimates the divergent beam coming from the focal point is positioned at a large distance behind that focal plane. In the case of this arrangement, the energy density of the laser beam in the vicinities of the second and third lens groups, i.e., on both sides of the focal point, should be less than the energy density of the incoming laser beam, which is intended to allow avoiding radiation-induced damage to those lenses, where the distance between these lenses and the focal point should not be allowed to become less than a minimum distance.
In many applications, in particular, applications in the field of the microlithographic fabrication of semiconductor devices and other types of microdevices, it is desirable to be able to switch between various illumination settings without having to move lenses over large distances. In addition, it is frequently desirable to have available a large expansion ratio, i.e., a large range of image-size variation, in order to be able to, for example, selecting widely differing conventional illumination settings. A boundary condition that is becoming increasingly important, particularly at short wavelengths, for example, 193 nm, 157 nm, or shorter wavelengths, is minimizing the total number of optical surfaces in the system in order to limit transmission losses. Furthermore, a telecentricity of the exit end (image end) of the zoom system is beneficial in order to allow adapting it to suit the optical systems that follow it in the optical train, particularly in the case of illumination systems on which exclusively angle-maintaining optical elements, such as rod-shaped light integrators, are arranged following their zoom system.